Lipoprotein lipase (LpL) is the principal enzyme responsible for hydrolysis of triglyceride in circulating lipoproteins. Changes in LpL activity are a basic mechanism used to modulate uptake of free fatty acids and perhaps fat-soluble vitamins by tissues. Regulation of LpL involves a number of post-secretory processes. By investigating these processes basic biological insights into the interaction of proteins with heparan sulfate proteoglycans (HSPGs) have been obtained. In this renewal, a series of experiments are proposed to understand how LpL is transferred from its sites of synthesis, principally adipocytes and myocytes, to the luminal surface of endothelial cells. Aim 1 is to determine the pathways required for LPL transport across endothelial cell monolayers. We have observed that LpL transcytosis across endothelial monolayers is reduced by RAP, the 39 kDa inhibitor of the LDL receptor related protein and other receptors in this family. In this Aim we will assess the role of these receptors and HSPGs in the transcytosis of normal and mutant LpL. The importance of this pathway and the association of LpL with proteoglycans will also be studied in vivo. Aim 2 proposes to study how LPL interaction with HSPGs regulates HSPG binding. Using these mice and new mice expressing a tethered dimer of mutated LpL, the importance of LpL-HSPG interactions will be studied. Aim 3 will study how the VLDL receptor and other RAP-sensitive receptor, and LpL monomizerization participate in the physiological regulation of LPL actions. We will assess the importance of LpL transcytosis pathways in wildtype and heterozygous LpL transcytosis pathways in wild type and heterozygous LpL knockout mice, the role of RAP-sensitive receptors in regulation of LpL activity with feeding/fasting, and modulation of tissue LpL in mice that cannot covert dimeric LpL to inactive monomers. These experiments will provide information about how a secreted protein transfers from the subendothelial space into the bloodstream and the importance of protein dimerization and HSPG binding in this process. Moreover by understanding the regulation of LpL activity, these investigations will may means to change human caloric and vitamin disposal; processes that are often abnormal in humans with lipoprotein disorders and diabetes.